Generally, computer controlled milling machines and similar mechanisms consist of very rigid rails to which a movable carriage is mounted, containing a head for mounting a cutter or other operating attachment. The object to be machined is mounted on a very rigid platform and the head is moved thereover. Such machines are so rigid that the head and operating attachment can be precisely positioned under the control of a computer.
Some machines, by the nature of their design, cannot position the head and operating attachment to a precise position and thus require supplemental alignment systems.
Other apparatuses have an operating head that is mounted on a carriage located on the end of a boom. The boom pivots in a horizontal plane about an axis on spaced circular rails. A laser alignment system senses any inaccuracies in the level of the rails and adjusts the operating head accordingly. However, this system assumes that the head is always properly positioned. This is because the boom and carriage are robust assemblies and only subject to rail inaccuracies.
Positioning systems also have been devised for resurfacing and repairing rails and guideways of large heavy machinery. A monorail assembly incorporating the milling head is assembled parallel to the rail. The straightness of the rail is determined by a laser measurement system. This information is fed to a computer and is used to align the monorail with the rail. The rail can then be machined to bring it back into tolerance. However, this apparatus requires a complex set up procedure and is only adapted to machine rails. It can not be used to machine molds and the like.